Subsetting a dataframe based on daily maxima

Question

I have an excel csv with a date/time column and a value associated with that date/time. I\'m trying to write a script that will go through this format (see below), and find 1) t

Answer 1

A solution using the plyr package, which I find very elegant for problems like this.

dat.str <- '         V1    V2 V3
1  5/1/2012  3:00  1
2  5/1/2012  6:00  2
3  5/1/2012  9:00  5
4  5/1/2012 12:00  3
5  5/1/2012 15:00  6
6  5/1/2012 18:00  2
7  5/1/2012 21:00  1
8  5/2/2012  0:00  2
9  5/2/2012  3:00  3
10 5/2/2012  6:00  6
11 5/2/2012  9:00  4
12 5/2/2012 12:00  6
13 5/2/2012 15:00  7
14 5/2/2012 18:00  9
15 5/2/2012 21:00  1'

dat <- read.table(textConnection(dat.str), row.names=1, header=TRUE)

library(plyr)
ddply(dat, .(V1), function(x){
   x[which.max(x$V3), ]
})